The invention relates to a method of producing thick welded steel pipes and an apparatus for effectively reducing the method to practice. The method and apparatus enables making thick welded steel pipes without the peaking amount on the edge preparation.
The UOE process is a known method for producing thick welded steel pipes. This method in general comprises preparing the steel plate by means of an edge planer to provide edges suitable to a diameter of a pipe to be formed and to the welding process, carrying out an edge-bending process on the edges, forming the steel into a U-shape through a U-ing press, and performing O-ing by means of an O-ing press on the U-shaped pipe-blank to form it into a pipe by means of an upper die and a lower die on a circular caliber. Subsequently, after a washing process, the O-shaped pipe-blank is subjected to a tack welding, and to an inner surface welding and an outer surface welding, followed by expanding the pipe by means of a mechanical expander.
The UOE process has been broadly used for producing steel pipes of large diameter. However, it has been inevitably involved with the occurrence of peaking in the case of a thick wall and high strength steel pipe such as deep-sea pipeline which has been recently desired, for example the thick steel pipe being API X65 and more than 2% in t/D (t: thickness and D: diameter).
The peaking is defined as a delta in FIG. 1 of projection from the regular circle Q, and not only should it be avoided in view of the product value but also it brings about instability on joining faces at welding after the O-ing, resulting in causing defects in the weld. Further, the peaking remaining after welding generates large angular distortion on the seam part during the sizing process during the expansion of the pipe and causes so-called expansion cracks.
Therefore, in the UOE piper production this peaking should be reduced as much as possible. However, due to under mentioned circumstances, a large amount of peaking has been inevitable in the prior art. That is, the UOE process treats as said above the edges of the plate with the edge bending process by the crimping press before entering the pipe-making stage. This bending basically depends upon the bending moment M.sub.0 =F.L between two points F and F as in FIG. 2. In order to bend the vicinity of the edges (L.fwdarw.0), a load F for obtaining the constant moment M.sub.0 becomes infinite theoretically. Therefore 1.0 to 1.5t of thickness t generally remains as non-processed, i.e., linear, thus causing peaking. FIG. 3 shows the peaking after O-ing which has been subjected to the edge-bending by means of the crimping press of 1500t. It is noted that the higher becomes the peaking, the higher are the thickness and the strength ("X65" and "X42" mean the strength grade of the pipe). As mentioned above, the main cause of the peaking is the undeformed straight part of a crimped edge. The peaking can be more or less reduced by a compressive process during O-ing. The mechanism of reducing the peaking is the buckling phenomena as shown in FIG. 4. In such case, a great load is required in order to bend the crimped edge because the moment arm (L) is very short. That is, the above mentioned method to reduce the peaking is not so effective although a great O-ing load is required. The conventional O-ing process is performed as shown in FIG. 5 by operating an upper die 1 and a lower die 2, having hemispherical upper and lower calibers 1a, 1b. In this case, as shown in FIG. 6 the steel plate 6 held between the upper and lower dies 1, 2 pressed by the O-press power P.sub.0, is bent by force F transmitted thereto in the circumferential direction, so that undeformed parts 61, 61 of the crimped edge are deformed along the die caliber 1a, 1b. The load P.sub.1 required for buckling is described by the following equation. EQU P.sub.1 =n.sub.1 .multidot..pi..sup.2 .multidot.E.multidot.t.sup.3 .multidot.L/h
wherein,
E is Young's modulus PA1 L is the length of the pipe PA1 h is the length of the undeformed part of crimped edge PA1 n.sub.1 is a constant PA1 .sigma..sub.z is deformation resistance of the material PA1 N.sub.2 is a constant
On the other hand, the following equation expresses the power P.sub.2 needed for the bending process before the compressing of the O-ing. EQU P.sub.2 =n.sub.2 .multidot.t.multidot.L.multidot..sigma..sub.z
wherein,
As noted from both expressions, the load P.sub.1 (buckling load of the undeformed parts) is proportional to the cube of the wall thickness t, and in comparing of P.sub.1 with P.sub.2, P.sub.1 is greater. FIG. 7 shows a representative example of a stroke vs load curve during the O-ing. The load of an area B corresponding to the compressing process is overwhelmingly larger than the load of an area A corresponding to the bending process, and therefore, in the prior art, it is impossible to reduce the peaking of the thick pipe, leaving aside the case of a thin pipe, in view of the facilities. The prior art has not been able to produce a thick pipe of API X65 where t/D exceeds 5%.
Such a problem is not particular to the UOE process, but exists in the production of thick wall steel pipe according to other processes, for example, the bending roll, cage forming, or bending press systems.
The present invention has been devised to eliminate those problems involved in the existing production of thick steel pipe.
It is a main object of the invention to offer a method which subjects to edge-bending the edge parts of the thick steel plate which have not been sufficiently processed with the edge-bending and remain as non-processed linear, in the O-ing press by means of a simple structure and at low force, thereby to control the peaking occurrence to be as little as possible.
It is another object of the invention to offer a thick steel pipe of large diameter and which is excellent in shape and of high quality by reducing the peaking to the minimum.
It is a further object of the invention to offer an apparatus which enables economically practicing the edge-bending of the steel plate by means of a simple facility.
Many other features and advantages of the invention will be apparent from the following description of the preferred embodiments of the invention as shown in the attached drawings.